AA-01 on a Daikin Altherma: Backup heater overheated or BUH power cable not connected
Short answer
On a Daikin heat pump, AA-01 means: Backup heater overheated or BUH power cable not connected. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Backup heater overheated or BUH power cable not connected.
Part involved: backup heater / thermal protector.
When it shows: Thermal protector is activated. Measured water temperature too high.
Likely causes:
- Water pressure is too low.; Water flow is too low.; Air in the water circuit.; Increased water temperature due to an external heat source.; Faulty backup heater thermal protector.; Faulty outlet water after backup heater thermistor.; Faulty backup heater.; Faulty hydro PCB.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: Manual reset via user interface and manual reset of backup heater thermal protector.
What you can check
- Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) and note it, with the date/time, for your installer.
- Check the heating-system water pressure reading on the gauge/display: Daikin operation manuals ask users to check it is above 1 bar. If it is below that, report it to your installer (do not open the unit).
- Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
- If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.
What the engineer will check
- Check the water pressure. . ; Check the water flow. . ; Purge the water circuit. . ; Check the water circuit for an external heat source. . ; Perform a check of the backup heater thermal protector. . ; Perform a check of the outlet water after backup heater thermistor. . ; Perform a check of the backup heater. . ; Perform a check of the hydro PCB. . ; | Troubleshooting.
Who to ask for: Heat pump installer with electrical competence.
Certification: Electrical competence to BS 7671 for heater/thermal cut-out work.
What you might notice
What owners and engineers report
12 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Unexpected backup / auxiliary heater use on the Altherma: 3 reports
- No hot water on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 34 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026